It is important to be able to remove certain spectral components from an image while leaving other spectral components. For example, in imaging of the eye by the usual magnetic resonance imaging (MRI) techniques, the optical nerve itself is covered by a layer of fat which obstructs the observation of the naked optical nerve. If the fat can be removed and just the water component of the image remains, then, a clear view is obtained of the optical nerve.
Also, at present, separate images of two different spectral components such as water and lipids within the patient are sometimes obtained. The separate images are important for diagnostic purposes; since, they supply the user with chemical information in addition to the morphological and anatomical information of conventional imaging.
Moreover by using an appropriate shift of one image in respect to the other, the two images can be combined in a manner which results in an image free of chemical shift artifacts. Presently, without taking proper steps, artifacts are caused by the different resonant frequencies of spectral components. For example hydrogen in fat has a different Larmor frequency than hydrogen in water. The Larmor frequency differences cause what are known as chemical shift artifacts.
A unique pair of inter-related sequences to obtain information on water and liquids in a patient was described in an article appearing in the journal Radiology entitled "Simple Protons Spectroscopic Imaging" by W. T. Dixon (0.153, 1984, PP189-194). In that article a method for encoding spectroscopic information and to produce clinical images is explained. The image produced differentiates between the water and fat intensities. The differentiation is done spectrally. The two mentioned patent applications are improvements on this method and also distinguish the water and fat components or in general spectral components spectrally. Disadvantages in the use of methods which spectrally differentiate between spectral components or chemically differentiated components are that a high degree of homogeneity of the field is required; or in the absence of the high degree of homogeneity, field maps are required showing the exact inhomogeneity of the magnetic field used during the test.
A review article entitled "Chemical Shift Imaging: A Review by L. Brateman" appeared in the American Journal of Radiology, volume 146, pp 971-980 (May 1986). It surveyed the prior art methods of chemical shift imaging which is defined in the article as "determining the spatial distribution of nuclei with a particular resonance frequency, such as water protons, rather than imaging the entire spectrum of resonance frequencies within a body".
Another method not directly related to the Dixon method is a chemical shift selection saturation method (see noted Brateman article and method 3). This too, however, requires a high degree of homogeneity or exact measurement of the imhomogeneity of the field in which the tests are conducted. The degree of homogeneity required has not been obtained in actual working systems. Phase mapping is time consuming and accordingly it is desirable to avoid the necessity of phase mapping and nonetheless, to provide data for separately imaging spectral components in the subjects being imaged.